The present invention relates to FM waveform generators and more particularly to direct digital FM waveform generators used to provide high range resolution in radar systems.
FM waveform generators produce a waveform having a frequency that varies from a start value to a stop value over the time span of each radar pulse transmitted for target detection. The variation of frequency with time is substantially linear to enable high range resolution to be achieved. With high range resolution, target distance is more accurately detected. It is useful in synthetic aperture radar and inverse synthetic aperture radar and in target identification modes.
Generally, the following equation defines range resolution: ##EQU1##
With greater FM bandwidth, namely with greater span between the start and stop frequencies, range resolution is made smaller to provide more accurate distance detection. For example, the range resolution can be approximately one foot for an FM bandwidth of 600 MHz.
The ideal linear FM waveform has quadratic phase. Errors in the waveform phase cause the FM waveform frequency to deviate from a linear variation. If the phase error is too great, false range responses, or excessive range sidelobes, are generated in the presence of a large range response. A phase error less than one to two degrees is normally required for sidelobe amplitudes below -35 dBc.
Waveform phase varies as a function of time in accordance with the following equation: EQU d.PHI./dt=2.pi.f EQU .PHI.=.intg.2.pi.fdt=.intg.2.pi.ktdt where k=linear FM slope [Hz/sec] EQU .PHI.=t.sup.2 .times..pi.k=.pi.kt.sup.2
A phase error can occur at any point in the time along the FM waveform where the actual .PHI. differs from .PHI. calculated in accordance with the above equation.
Linear FM waveform parameters, especially useful to imaging radars, are starting phase of the linear FM waveform, frequency offset, and linear FM slope. If these parameters are programmable and precisely controllable, hardware complexity is reduced in imaging radars.
A first technique for achieving a linear FM waveform with wide bandwidth and good range sidelobes (&lt;-35 dB) uses a sampled phase-locked loop, as disclosed for example in U.S. Pat. No. 4,160,958. The sampled phase-locked loop provides excellent range sidelobe performance at bandwidths up to 1 GHz.
Linear FM waveforms can also be generated by a direct digital synthesis (DDS) technique. With the use of GaAs logic and digital-to-analog converters (DACs), bandwidths up to 600 MHz or more are feasible with DDS. Recent literature on this subject includes the following:
1. "A direct digitally synthesized exciter achieving near theoretical performance for an operational SAR system", Gallaway et. al., 88CH2572-6/88/0000-0022 1988 IEEE.
2. "Model 2040 800 Mpoint/second polynominal waveform synthesizer." Analogic product data sheet. 1989.
3. "32-bit DDS Phase Accumulator, 1.0 GHz clock rate." GaAs IC data book and designers Catalog. Gigabit. 1988. p1-153.
4. "Model TQ6112 8 bit, 1 GHz DAC". Triquint Data Sheet. 1987.
DDS has several advantages when compared to the phase-locked loop approach: 1. reduced hardware complexity; 2. faster waveform reset time; and 3. larger linear FM slopes. The significance of these advantages varies according to the radar application.
The typical prior art DDS FM waveform generator employs digital circuitry to generate a linear FM waveform that drives a DAC. The digital circuitry normally is either a programmable read only memory (PROM) which has stored samples of the waveform, or a phase accumulator and PROM which generate the waveform in real time. The waveform output from the DAC is smoothed by filtering and then frequency offset to the microwave range in one or more mixer stages, each having output filtering that rejects unwanted mixer products. Finally, the frequency offset waveform is frequency multiplied to generate the output linear FM waveform.
The FM bandwidth generated at the DAC output is limited to the Nyquist frequency which equals one half of the clock rate in accordance with Nyquist sampling theory. The DAC output is a staircase approximation to the desired linear FM waveform, and it includes periodic higher frequency extensions. An ideal or "brickwall" low-pass filter would remove undesired higher frequency spectra, but the use of real filters results in a tradeoff between the maximum generated bandwidth and the attenuation of the desired and undesired spectra.
Actual low-pass filters also introduce phase errors on the linear FM waveform that can further reduce bandwidth. The additional circuitry after the low-pass filter for mixing, filtering and frequency multiplication expand the linear FM bandwidth beyond that obtainable at the maximum clock rate of the DDS generator. The added mixing and filtering circuitry contribute to circuit complexity and introduce more phase error.
The present invention is directed to a DDS FM waveform generator having simpler more economic circuit structure that operates with reduced phase error and provides other improved features thereby enabling better radar system performance including higher radar resolution and greater radar reliability.